This invention relates to a control system of an ozonizer system capable of operating the ozonizer system at high efficiencies.
Since ozone has a strong oxidizing power it has been used in many applications such as treatment of waste water containing organic matters, decoloring of colored waste water, sterilization and deodorization of aqueduct water and denitrification of engine exhaust gas or flue gas containing nitrogen oxides NOx. As a consequence, various types of industrial ozonizers have been developed in recent years wherein air or oxygen is used as the raw material gas for producing ozone.
Generally, an ozonizer comprises a plurality of unit ozone generators each including an inner glass pipe and an outer metal pipe and the quantity of the ozone generated is determined by the number of the unit ozone generators. From the standpoint of handling and machining, the maximum dimensions of a practical unit ozone generator are: a diameter of about 80 mm and a length of about 2000 mm. These data determine the area of electric discharge and hence the maximum value of the amount of ozone generated. Accordingly, the capacity of an ozonizer system can be increased either by increasing the number of the unit ozone generators or by grouping into a module a relatively small number of unit ozone generators and by parallelly operating a plurality of such modules.
Generally, the module type ozonizer system is more advantageous than single ozonizer having the same capacity from the standpoint of utilization factor. Denoting the utilization factor by A, and the percentages of fault and repair of the module by .lambda. and .mu. and asscerning that .lambda. and .mu. are equal for each module, then a relation A = .mu./(.lambda. + .mu.) holds for each module. If P modules among a total of N modules are in an operable condition under a partial load condition under which it is possible to produce a quantity of ozone consistent with the demand the utilization factor can be expressed by the following equation ##EQU1## WHERE I REPRESENTS AN INTEGER SMALLER THAN OR EQUAL TO N. To simplify the description, suppose now that N=3 and P=2, then ##EQU2## Where the mean time between failures (MTBF) is equal to 1,000 hours and the mean time to repairs (MTTR) is equal to 10 hours, the utilization factor is calculated as A.sub.2/3 = 0.9997 since .lambda. = 0.0011/hour and .mu. = 0.11/hour. In the case of a single ozone generator having the same capacity as a group of modules since A.sub.1/1 = .mu./.lambda. + .mu., the utilization factor is equal to A.sub.1/1 = 0.9901, showing that the utilization factor of the module type ozonizer system is higher than that of a single ozone generator.
The efficiency of ozone generation of an ozonizer utilizing air as the raw material is about 5% of the electric power supplied to the ozone generator and remaining 95% is converted into heat. An increase in the temperature of the ozone generator and of the raw material air decreases the efficiency of ozone generation and the insulating strength of the dielectrics utilized in the ozone generator. For this reason, it is necessary to cool and dry the raw material air and to cool the ozone generator. In some cases, the power required for such cooling amounts to about 1/3 of the total power of the ozonizer system.
While the quantity of the ozone generated increases in proportion to the applied voltage or frequency since these electric quantities are proportional to the electric power, the quantity of heat generated increases with these electric quantities. Especially, in the case of a high frequency discharge, although the quantity of the ozone generated increases, the temperature increases greatly due to the increase in the dielectric loss so that if cooling is not sufficient, insulation breakdown occurs. As the temperature increases, decomposition of the ozone generated becomes remarkable, thereby decreasing the yield of ozone. Where the ozone demand varies with time due to the variation of the load of the treating apparatus utilizing ozone, unless the electric power supplied to ozone generators constituting an ozonizer system and to such peripheral apparatus as the apparatus for feeding, cooling, drying and distributing the raw material air, and a device for cooling the cooling water of such apparatus is controlled precisely, the quantity of the ozone generated per unit power that is the yield of the ozone decreases, thus increasing the operating cost of the ozonizer system. In the prior art ozonizer system control was made without considering these problems so that energy loss is large and the operation cost is high.